The present invention relates to a system which includes a permanent magnet displacement transducer, and more particulary to a transducer wherein a permanent magnet is coupled to a movable member such as a diaphragm that is responsive to pressure or differential pressure, etc. The system of the present invention is particularly useful in hostile environments.
Measurements in hostile environments typically are limited by the environmental effects on sensors and transmission systems.
Reliability of these measurements requires substantial margins against potential failures to achieve desired dependability goals. The high costs of programs to provide statistical substantiation of reliability objectives in these severe environments favors the use of a minimum number of components and materials such that existing experience data bases can substantiate reliability goals and margins in design.
A very limited experience data base exists in the area of materials and components subject to combined effects of temperature, chemistry and (nuclear) radiation [synergistic effects]. A combination of radiation and "high" temperatures have "annealing" effects on some materials. No such benefits exist with radiation at low temperatures. As a result performance predictions by analysis are suspect except where significant margins are known to exist. Further, the substantiation of reliability goals for a wide range of combinational situations makes testing very costly.
The invention described employs a minimal number of materials and their associated properties in a system configuration controlled by design for tolerance to severe environments and remote interpretative electronics in a controlled environment coupled via a two conductor cable. The latter may typically be existing cabling as currently employed by "current loop" transmitters. The invention may therefore be economically applied as a modular replacement for existing systems employing greater complexities in hostile environments.
In a nuclear power plant, a reactor vessel and associated structures which comprise a nuclear steam supply system are located within a containment building. The nuclear steam supply system typically consists of several closed loops which convey hot water from the reactor vessel to steam generators for extraction of energy and then return the water to the reactor vessel for re-heating. In order to properly control and monitor the reactor's operation, it is necessary to provide instruments which measure the various process parameters of the coolant systems. The containment building, as its name implies, is provided in order to prevent fission products from escaping into the external environment in the event of an accident. Accordingly, process measurement instruments within the containment building must be impervious to extremely adverse conditions such as high radiation levels and temperature.
Because of the high temperature and radiation which might exist within the containment building during an accident pressure transducers that are normally used employ physically rugged electromechanical devices to detect displacement of a diaphragm. The transducing sensor devices normally require in excess of two conductors for operation, but are typically supported by local "current loop" signal conditioning transmitters which effectively reduce the number of conductors required for each transducer to two. During construction of a nuclear plant, pairs of conductors are installed to electrically connect the transmitters through the containment boundary to instruments within a control room, which is positioned outside the containment building. Two conductor "current loop" transmitters employ specially designed electronics which are qualification tested to assure performance under accident conditions following a normal service lifetime.
Alternatively pressure transducers are used which require three or more conductor cabling systems to remote interpretative electronics systems.
Most existing nuclear power plants in the United States employ pressure transducer systems having two conductor "current loop" transmitters. Since the amount of wiring through the containment boundary that is established at the time of plant construction is limited, and since installation of additional wiring at a later time is extremely expensive, when existing pressure transducer systems are replaced it is economically desirable to use replacements that do not require additional wiring. In view of the high cost of qualification testing it would also be desirable to use replacement systems which do not require transducer-support by electronics within the containment building, since electronics within the containment building must be designed to withstand high temperature and radiation.